Waste treatment process and apparatus

ABSTRACT

A process for the treatment of waste, the process comprising either a gasification step or a pyrolysis step to produce an offgas and a non-airborne, solid char material; followed by a plasma treatment step. An associated apparatus having a plasma treatment unit which is separate from the gasification unit or pyrolysis unit.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/577,336, filed Apr. 16, 2007, based on PCT/GB2006/002409, filed Jun.29, 2006, claiming priority from GB application 0513299.8 filed Jun. 29,2005 and 0604691.6 filed Mar. 8, 2006, the entire disclosures of whichare expressly incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a process for treating waste,particularly municipal waste.

BACKGROUND ART

Municipal waste has traditionally been disposed of in landfill sites.However, the environmental hazards of doing so are becoming a majorconcern and therefore an effort has been made in recent years to developwaste-treatment processes that reduce the volume of the waste materialand the amount of potentially environmentally hazardous constituents inthe treated material.

Processes that have been developed to treat waste include combustionsystems, in which the waste is thermally processed with stoichiometricor excess amounts of oxygen. The process is normally carried out in air.Examples of combustion systems include: mass-fired combustion systems,refuse derived fuel (RDF) combustion systems, in which the RDF istypically burnt on a travelling grate stoker, and fluidised bedcombustion.

Another method of processing waste involves using pyrolysis, i.e.pyrolysing the waste in a pyrolysis unit. The term pyrolysis means, inthe field of waste treatment, the thermal processing of waste in theabsence of oxygen. Generally pyrolysis processes are endothermic and sorequire the input of thermal energy for the pyrolysis to continue. Thiscontrasts with combustion, which is an exothermic process, and as suchdoes not require the additional input of heat once the combustion hasbeen initiated. The pyrolysis process converts many of the organicconstituents found in waste to gaseous, liquid and solid fractions usinga combination of thermal cracking and condensation reactions. Pyrolysisgenerally results in three products: a gas stream, primarily containinghydrogen, methane, carbon monoxide, carbon dioxide and other gases; aliquid fraction containing a tar or oil containing acetic acid, acetone,methanol, and complex oxygenated hydrocarbons; a char, consisting ofalmost pure carbon, plus any originally inert material originallypresent in the solid waste. Pyrolysis is a process that is used in theindustrial production of charcoal from wood, coke and coke gas fromcoal, and fuel gas and pitch from heavy petroleum fractions. However,its use in the processing of solid waste has not been successful, one ofthe reasons for which is that the system requires a consistentfeedstock, which is difficult to obtain from municipal waste.

A third method for processing waste involves the gasification of thewaste. Gasification is the partial combustion of a material, where theoxygen in the gasification unit is controlled such that it is present ata sub-stoichiometric amount, relative to the waste material.Gasification of waste containing carbonaceous components results in acombustible fuel gas rich in carbon monoxide, hydrogen and somesaturated hydrocarbons, principally methane. There are five basic typesof gasifier: vertical fixed bed gasifier, horizontal fixed bed gasifier,fluidised bed gasifier, multiple hearth gasifier and rotary kilngasifier. The first three are in most common use.

Gasification, while being moderately successful in combusting themajority of waste, nevertheless produces a gas that contains uncombustedparticulates, low volatility tarry species and airborne compounds.Additionally, although much of the waste is combusted to either a gas orairborne particles, the gasification process still often results in a‘char’, i.e. a solid material that contains constituents that will notreadily combust or vaporise under the operating conditions of thegasification. The char commonly contains hazardous heavy metal and toxicorganic species, which must be disposed of carefully, adding to the costof the overall waste treatment process. It will be appreciated thatthere is a desire to reduce the amount of solid waste that results froma waste-treatment process, and also reduce the amount of hazardousmaterials in the treated waste.

It has also been found that, if the gas that results from thegasification of waste (termed an ‘offgas’) is used in a gas engine orgas turbine, the airborne particulates and tarry hydrocarbon moleculeshave a tendency to clog the gas turbine or engine. The gas is thereforenot considered to be sufficiently ‘clean’ and even if the offgasproduced by the gasification were to be used, the turbine would requirefrequent cleaning and maintenance and/or the introduction of anadditional costly cleaning stage to remove the tarry products.

There is therefore a desire for a process that will overcome, or atleast mitigate, some or all of the problems associated with the methodsof the prior art.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a process for thetreatment of waste, the process comprising

a gasification step comprising treating the waste in a gasification unitto produce an offgas and a char, and

a plasma treatment step comprising subjecting the offgas and the charproduct to a plasma treatment in a plasma treatment unit. The offgastypically will contain uncombusted solid particles and tarry species.

The first aspect may provide a process for the treatment of waste, theprocess comprising

(i) either

-   -   (a) a gasification step comprising treating the waste in a        gasification unit in the presence of oxygen and steam to produce        an offgas and a char, or    -   (b) a pyrolysis step comprising treating the waste in a        pyrolysis unit to produce an offgas and a char; and

(ii) a plasma treatment step comprising subjecting the offgas and thechar to a plasma treatment in a plasma treatment unit in the presence ofoxygen and, optionally, steam.

The first aspect may provide a process for the treatment of waste, theprocess comprising

(i) subjecting the waste to microbial digestion, then

(ii) either

-   -   (a) a gasification step comprising treating the microbially        treated waste in a gasification unit to produce an offgas and a        char, or    -   (b) a pyrolysis step comprising treating the microbially treated        waste in a pyrolysis unit to produce an offgas and a char; and

(iii) a plasma treatment step comprising subjecting the offgas and thechar to a plasma treatment in a plasma treatment unit.

In the presence of oxygen and steam” indicates that both oxygen gas andsteam are present in the gasification unit and/or the plasma treatmentunit. Other gases may also be present. Oxygen may be provided as oxygengas, in a mixture of gases (for example air), and/or in anoxygen-containing compound.

“Steam” includes water in the gaseous form, vapour and water suspendedin a gas as droplets. Preferably, the steam is water having atemperature of 100° C. or more. Water, which will be converted to steam,may be introduced into the gasification unit and/or plasma treatmentunit in the form of liquid water, a spray of water, which may have atemperature of 100° C. or less, or as vapour having a temperature of100° C. or more; in use, the heat in the interior of the gasificationunit and/or plasma treatment unit ensures that any liquid water, whichmay be in the form of airborne droplets, is vaporised to steam.

The second aspect may provide an apparatus for carrying out the processof the present invention, the apparatus comprising:

(i) a gasification unit or pyrolysis unit and

(ii) a plasma treatment unit,

wherein the gasification unit has inlets for oxygen and steam and theplasma treatment units has an inlet for oxygen and optionally an inletfor steam.

The second aspect may provide an apparatus for carrying out the processof the present invention, the apparatus comprising:

(i) a microbial digestion unit,

(ii) a gasification or pyrolysis unit, and

(iii) a plasma treatment unit.

Preferred features of the present invention are described in thedependent claims and in the Detailed Description below.

The present invention will now be further described. In the followingpassages different aspects of then invention are defined in more detail.Each aspect so defined may be combined with any other aspect or aspectsunless clearly indicated to the contrary. In particular, any featureindicated as being preferred or advantageous may be combined with anyother feature or features indicated as being preferred or advantageous.

BRIEF DESCRIPTION OF THE FIGURES

The following figures are provided by way of example and shownon-limiting embodiments of the present invention.

FIG. 1( a) to (c) show schematic drawings of a plasma furnace having twoelectrodes in three possible configurations.

FIG. 2 shows a preferred embodiment of the process of the presentinvention.

FIG. 3 shows an embodiment of the apparatus of the present invention,including a fluid bed gasifier (1) and a plasma furnace (4).

FIG. 4 shows in more detail the plasma furnace of FIG. 3.

FIG. 5 shows a further preferred embodiment of the process of thepresent invention.

DETAILED DESCRIPTION

Having regard to the problems associated with gasification processes, aproposed solution considered by the present inventors was to use aplasma treatment in place of the gasification treatment. The inventorsfound, however, that the amount of energy required to gasify the organicfraction of the waste material in the plasma unit was very high and onlyrelatively small volumes of solid waste could be treated at any onetime. As such, the treatment of unprocessed waste using plasma was foundnot to be economically viable. The inventors have now found, however,that by first treating the waste in a gasification unit, followed bytreatment in a plasma unit, a number of advantages over the prior artprocesses can be obtained. In particular, this combination has beenfound to be surprisingly energy efficient. It has also been found thatthe combination of the gasification treatment and the plasma treatmentresults in a relatively clean syngas (containing very low concentrationsof airborne particulates), very low amounts of hazardous tar and heavymetal species and smaller amounts of solid material in the cleaned gasproduct.

The treatment of the offgas in a plasma unit has been found tosignificantly reduce the number of airborne particulates and tarryhydrocarbon compounds, which have a tendency to create fouling problemsif used in a turbine. The treatment of the char in the plasma unit hasbeen found to convert much of the char material to a gas, and, inparticular, a gas that has a relatively low content of airborneparticulates and tarry gaseous hydrocarbons, which could clog a turbine.The plasma also has the advantage that various environmentally harmfulairborne particulates and gases are degraded to less harmful speciesduring the plasma process.

Preferably, the process involves introducing a waste material, i.e. awaste feedstock, that is substantially homogenous to the gasificationunit. This has been found to improve the efficiency of the treatmentprocess as a whole. The waste feedstock may have been pre-treated toincrease its homogeneity prior to introduction to the gasification unit.“Homogenous” indicates that the waste should have one or more propertieswhich do not vary to a great extent throughout the bulk of the waste orfrom batch to batch, if the waste feedstock is fed in batches to thegasifier; hence the value of the property in question does not vary to agreat extent as the waste is fed to the gasification unit. Suchproperties that preferably do not vary to a great extent include thecalorific value, the size of constituents, moisture content, ashcontent, and density of the waste material. Preferably one or more ofthese properties varies by 20% or less, preferably 15% or less, morepreferably 10% or less. Preferably, the calorific value and the moisturecontent of the waste being fed to the gasifier are relatively consistentduring the process.

The consistency of the property/properties of interest may be measuredby taking samples of the same weight from either (i) a given number ofbatches of the feedstock fed to the gasifier over a period of time (ifthe feedstock is fed batch-wise to the gasifier) or (ii) at givenintervals of time if the feedstock is fed substantially continuously tothe gasifier. Sampling methods known to the skilled person may be usedto measure the consistency of the waste feedstock.

For example, over a period of 1 hour of running the process, thecalorific value of samples of the waste (of the same weight, e.g. 1 kgor 10 kg) being fed to the gasifier taken at regular (e.g. 5 to 10minutes or 3 to 4 hours) intervals preferably varies by 20% or less,more preferably 15% or less, most preferably 10% or less. On an absolutescale, the waste feedstock typically has a mean calorific value ofaround 15 MJ/kg, and preferably has a (+/−) variation from the meancalorific value of less than 3 MJ/kg, preferably less than 1.5 MJ/Kg.The moisture content of the waste feedstock is preferably as low aspossible, as discussed in more detail below. The average (mean)calorific value of the waste feedstock (which may be calculated from avariety of samples taken at regular intervals, as described above) ispreferably 11 MJ/Kg or above, more preferably 13 MJ/Kg or above, mostpreferably 15-17 MJ/Kg.

The waste feedstock, i.e. the waste fed to the gasifier (which maycomprise refuse derived fuel), preferably has a moisture content of 30%or less by weight, preferably 20% or less by weight, more preferably 15%or less by weight. The moisture content of the waste feedstockpreferably varies by 10% or less, more preferably by 5% or less. Themoisture content of the waste feedstock may be controlled usingprocesses known to those skilled in the art, such as drying, or by usingthe microbial digestion processes described herein. Typical moisturecontent of refuse derived fuel may be in the range of 20 to 40% byweight. Preferably, the moisture content of the refused derived fuel isreduced to the preferred amounts for the waste feedstock describedabove.

The process may further comprise the step of drying the waste before itstreatment in the gasification or pyrolysis step. The waste may be driedby using the heat produced in any of the other steps of the process,such as heat from the pyrolysis, gasification and/or plasma treatmentsteps. Heat may be transferred to the waste for the purposes of dryingby contacting it with heated air or steam, which may in turn have beenheated from the heat produced in any of the other steps. The waste maybe dried by blowing heated air or steam over or through the waste.

The waste feedstock preferably contains a high proportion (preferably85% or more of the number of particles, more preferably 95% or more ofthe number of particles) of particles having a particle size of 50 mm orless. A particle's size is measured across the particle at its largestdimension. Preferably the feedstock contains 50% or more (by number) ofparticles having a particle size of 30 mm or less.

A typical analysis of the waste feedstock content would be as follows:

-   -   Gross calorific value: 13.2 MJ/Kg    -   Moisture: 25%    -   Ash: 13.05%    -   Fixed carbon: 12.17%    -   Volatile matter: 49.78%    -   Particle size: 85%<50 mm

Various processes may be used to homogenise various properties of thewaste material, for example: microbial digestion, picking, shredding,drying, screening, mixing and blending. Of these, microbial digestion ispreferred and this process is explained in more detail below.

A suitable waste material for use in the gasification step was analysedin two forms, each form having a different moisture content butotherwise the same components in the same proportions. The wastematerial contained the components shown in Table 1 below. The fourthcolumn gives the weight % of the components for each sample in theabsence of moisture. The gasification unit is preferably adapted togasify the waste having the content as given in the table below. Theelemental analysis (ultimate analysis) of the waste is given in Table 2below.

TABLE 1 Weight % Weight % Weight % (including 12% (including 25%(excluding Moister in the water in the Water from Component total -form 1) total - form 2) the total) Paper and Card 36.19 30.84 41.12Plastic Film 15.2 12.96 17.27 Dense Plastic 2.59 2.21 2.94 Misc. 6.645.65 7.54 Combustibles Misc. Non- 2.19 1.87 2.49 Combustibles Glass 3.653.11 4.15 Ferrous Metals 1.19 1.01 1.35 Non-Ferrous 0.28 0.24 0.32Metals Vegetable and 8.86 7.39 9.86 Putrescible matter Textiles 4.623.94 5.25 Nappies and 6.78 5.78 7.71 Pads Moister 12.00 25.00 0 Total(wt %) 100 100 100

A waste material having been thermally dried may have a moisture contentin the range 10-16 wt % of about 12% or less: the above form I of thewaste is therefore representative of thermally dried waste. A wastematerial having been dried by a so-called ‘MBT’ (Mechanical BiologicalTreatment, such as rotary aerobic digestion) may have a moisture contentof about 25% or less: the above form II is therefore representative ofwaste that has been subjected to MBT.

TABLE 2 (Ultimate analysis of waste from Table 1 containing 25% moistureby weight) Ash (other Moisture C H O S N Cl elements) Content 36.9 4.924.12 0.15 0.5 0.5 8.03 24.9 100

The elemental amounts of H and O in Table 2 are from the theoreticallydry components.

The process according to the present invention comprises a gasificationstep. The gasification step may, for example, be carried out in avertical fixed bed (shaft) gasifier, a horizontal fixed bed gasifier, afluidised bed gasifier, a multiple hearth gasifier or a rotary kilngasifier.

It should be noted that a horizontal fixed bed gasifier may otherwise bereferred to in the prior art as a starved air combustor (incinerator),controlled air combustor, pyrolytic combustor, or a modular combustionunit (MCU).

A horizontal fixed bed gasifier generally comprises two sections: aprimary combustion chamber and a secondary combustion chamber. In theprimary chamber, waste is gasified by partial combustion undersub-stoichiometric conditions, producing low-calorific gas, which thenflows into the secondary combustion chamber, where it is combusted withexcess air. The secondary combustion produces high-temperature (650 to870° C.) gases of complete combustion, which can be used to producesteam or hot water in an optionally attached waste boiler. Lowervelocity and turbulence in the primary combustion chamber minimize theentrainment of particulates in the gas stream, leading to lowerparticulate emissions than conventional excess-air combustors.

Preferably, the gasification step is carried out in a fluid bedgasification unit. Fluid bed gasification has been found to process thewaste feedstock more efficiently than the other gasification processesavailable. The fluid bed technique permits very efficient contacting ofthe oxidant and waste feed streams leading to rapid gasification ratesand close temperature control within the unit.

A typical fluid bed gasification unit may comprise a vertical steelcylinder, usually refractory lined, with a sand bed, a supporting gridplate and air injection nozzles known as tuyeres. When air is forced upthrough the tuyeres, the bed fluidises and expands up to twice itsresting volume. Solid fuels such as coal or refused derived fuel, or inthe case of the present invention, the waste feedstock, can beintroduced, possibly by means of injection, into the reactor below orabove the level of the fluidised bed. The “boiling” action of thefluidised bed promotes turbulence and transfers heat to the wastefeedstock. In operation, auxiliary fuel (natural gas or fuel oil) isused to bring the bed up to operating temperature 550° C. to 950° C.,preferably 650° C. to 850° C. After start-up, auxiliary fuel is usuallynot needed.

Preferably the gasification unit, most preferably the fluid bedgasification unit, will be a vertical, cylindrical vessel, which ispreferably lined with an appropriate refractory material, preferablycomprising alumina silicate.

In a fluid bed gasification unit, the distance between the effectivesurface formed by the particles of the fluid bed when fluid (i.e. whengas is being fed through the particles from below) and the top of theunit is called the “free board height”. In the present invention, thefree board height, in use, will preferably be 2.5-5.0 (more preferably3.5 to 5.0) times the internal diameter of the unit. This geometricconfiguration of the vessel is designed to permit adequate residencetime of the waste within the fluid bed to drive the gasificationreactions to completion and also to prevent excessive carry over ofparticulates into the plasma unit. The gasification unit will preferablyemploy a heated bed of ceramic particles suspended (fluidized) within arising column of gas. The particles may be sand-like. The particles maycomprise silicon oxide.

Preferably, the waste will be fed continuously to the gasification unitat a controlled rate. If the gasification unit is a fluid bedgasification unit, preferably the waste is fed either directly into thebed or above the bed.

Preferably, the waste feed will be transferred to the gasifier unitusing a screw conveyor system, which enables continuous addition ofwaste. The waste feed system may incorporate an air lock device, suchthat the waste can be fed into the gasification unit through the airlock device to prevent air ingress or gas egress to/from the interior ofthe gasifier unit. The waste is preferably fed through the airlockdevice with additional inert gas purging. Air lock devices are known tothe skilled person.

During the gasification process, the gasification unit should be sealedfrom the surrounding environment to prevent ingress or egress of gasesto/from the gasification unit, with the amount of oxygen and/or steambeing introduced to the gasification unit as required in a controlledmanner.

If the gasification unit is a fluid bed gasification unit, preferablyoxidants comprising oxygen and steam are fed below the bed, which may bethrough a series of upward facing distribution nozzles.

Preferably, the gasification is carried out in the presence of steam andoxygen. As mentioned above, water, which will be converted to steam, maybe introduced into the gasification unit in the form of liquid water, aspray of water, which may have a temperature of 100° C. or less, or asvapour having a temperature of 100° C. or more. In use, the heat in theinterior of the gasification unit ensures that any liquid water, whichmay be in the form of airborne droplets, is vaporised to steam.Preferably the steam and oxygen will be closely metered to the unit andthe rate of waste feed adjusted to ensure that the gasifier operateswithin an acceptable regime. The amount of oxygen and steam introducedto the gasification unit relative to the amount of waste will depend ona number of factors including the composition of the waste feed, itsmoisture content and calorific value. Preferably, the amount of oxygenintroduced to the gasification unit during the gasification step is from300 to 350 kg per 1000 kg of waste fed to the gasification unit.Preferably, the amount of steam introduced to the gasification unit isfrom 0 to 350 kg per 1000 kg of waste introduced to the gasificationunit, optionally from 90 to 300 kg per 1000 kg, of waste or 120 to 300kg per 1000 kg of waste, most preferably 300-350 kg of waste, if thewaste contains less than 20% (optionally less than 18%) by weightmoisture. If the waste contains 20% or more (optionally more than 18%)by weight moisture, preferably the amount of steam introduced to thegasification unit is from 0 to 150 kg per 1000 kg of waste. Typicaladdition amounts of oxygen and steam oxidants for the waste given abovein Table 1 are given below in Table 2.

The gasification unit will preferably comprise a fossil fuelled underbedpreheat system, which will preferably be used to raise the temperatureof the bed prior to commencement of feeding to the unit.

Preferably the gasification unit will comprise multiple pressure andtemperature sensors to closely monitor the gasification operation.

For the waste material having the composition given in Table 1(containing either 12% or 25% water), the addition rate of oxygen andsteam will preferably be in the range as indicated in Table 2 below.

TABLE 2 Typical relative addition amounts of oxygen and steam oxidantsRDF 12% moisture* RDF 25% moisture* Relative oxygen 300-350 300-350addition amount (kg per 1000 kg waste) Relative steam  90-3000  0-100addition amount (kg per 1000 kg waste) *Based on composition of wastefeed (the refuse derived fuel) given in table 1

Preferably the waste will be gasified in the gasification unit at atemperature greater than 650° C., more preferably at a temperaturegreater than 650° C. up to a temperature of 1000° C., most preferably ata temperature of from 800° C. to 950° C. If a fluid bed gasifier isemployed in the present invention, preferably the bed temperature ismaintained in the range of from 650-900° C., more preferably in therange of from 750-950° C. and most preferably in the range of from800-850° C.; this is generally suitable for all waste that does not havea high potash content and no agglomeration (sintering) of the fluid bedparticles is observed.

The maximum temperature that can be employed in the fluidised bed of afluidised gasification unit is dependent on the composition of the ashcontent of the fuel being treated. In particular, some biomass materialsare high in potash, soda and other species that form low melting pointeutectics. For these waste containing one or more of these materials, itis especially important to ensure that the temperature of the bed iskept below the sintering temperature of the ash within the waste (whichmay be as low as .about.650° C. in certain cases) to avoid coagulationof the fluid bed particles. The temperature of the fluidised bed may bemaintained by controlling the amount of oxidant fed to the gasifierrelative to the amount of the solid fuel.

In the fluid bed gasifier, preferably the zone above the fluid bed(sometimes termed the freeboard) may be a higher temperature than thefluid bed. The temperature of the zone above the fluid bed is preferablyin the range of from 800-1000° C.

Fluid bed gasification systems are quite versatile and can be operatedon a wide variety of fuels, including municipal waste, sludge, biomassmaterials, coal and numerous chemical wastes. The gasification step ofthe process of the present invention may comprise using a suitable bedmedia such as limestone (CaCO₃), or, preferably, sand. During operation,the original bed material may be consumed, and may be replaced byrecycled graded ash (Char) material from the gasification stage.

Preferably, the gasification unit and the plasma treatment unit areintegrated and will typically be in fluid connection. ‘Fluid connection’indicates that a conduit is provided for transporting the products ofthe gasification unit to the plasma treatment unit. Preferably, thewhole process is an integrated process, in that all the steps arecarried out on one site and means are provided to transport the productsfrom each step to the next. Each step is preferably carried out in aseparate unit. In particular, the gasification and the plasma treatmentare preferably carried out in separate units, to allow the conditions ineach unit to be varied independently. Preferably, means are provided fortransporting the products of the gasification step from the gasificationunit to the plasma treatment unit.

Pyrolysis, as a process, and pyrolysis units are conventional and knownto those skilled in the art and are commercially available.

In an alternative embodiment, the plasma treatment may be conducted intwo units to separately treat the solid char and the gasifier off-gasstreams.

The process according to the present invention comprises a plasmatreatment step. Preferably, the plasma treatment is carried out in thepresence of an oxidant. Preferably, the amount of oxidant is controlled.More preferably, the amount of oxidant is controlled such that that thegaseous hydrocarbons (including low volatility, tar products), theairborne carbon particulates, carbon contained in the char and part ofthe carbon monoxide is converted to carbon monoxide and carbon dioxide,preferably such that the ratio of the CO/CO.sub.2 after the plasmatreatment stage is equal or greater than the gas exiting the gasifierunit. Preferably, the plasma treatment is carried out on the char untilsubstantially all of the carbon content in the char has been convertedto gas or airborne species.

Preferably, the oxidant is oxygen or oxygen and steam. Preferably, theplasma treatment is carried out in the presence of oxygen and steam. Asmentioned above, water, which will be converted to steam, may beintroduced into plasma treatment unit in the form of liquid water, aspray of water, which may have a temperature of 100° C. or less, or asvapour having a temperature of 100° C. or more. In use, the heat in theinterior of the gasification unit and/or plasma treatment unit ensuresthat any liquid water, which may be in the form of airborne droplets, isvaporised to steam.

Preferably, the ratio of oxygen to steam is from 10:1 to 2:5, by weight.Preferably, the plasma treatment of the waste is carried out at atemperature of from 1100 to 1700° C., preferably from 1100 to 1600° C.,more preferably from 1200 to 1500° C.

The plasma unit in operation will generally contain a melt phase. Thetemperature of the melt phase in the plasma unit will preferably be1150° C. or more, preferably of from 1150° C. to 1600° C.

Preferably, the amount of oxygen introduced to the plasma unit for every1000 kg of waste initially introduced into the gasification unit is from15 to 100 kg, preferably from 25 to 80 kg. Preferably, the amount ofsteam introduced to the plasma unit for every 1000 kg of waste initiallyintroduced into the gasifier is from 0 to 50 kg, preferably 0 to 30 kg.

For the waste material having the composition given in Table 1(containing either 12% or 25% water), the addition rate of oxygen andsteam to the plasma converter will preferably be in the range asindicated in Table 3 below.

Typical relative addition amounts of oxygen and steam oxidants to theplasma converter unit RDF 12% moisture* RDF 25% moisture* Relativeoxygen 25-80 25-80 addition amount (kg per 1000 kg waste) Relative steam 0-30  0-30 addition amount (kg per 1000 kg waste) *Based on compositionof waste feed (the refuse derived fuel) given in table 1

Preferably, the plasma treatment of the waste is carried out in thepresence of a plasma stabilizing gas. Preferably, the plasma stabilizinggas is selected from one or more of nitrogen, argon, hydrogen, carbonmonoxide, carbon dioxide and steam.

Preferably, water, which will be converted into steam, is introducedinto the plasma treatment unit in the form of a spray of water having atemperature below 100° C. There are two main advantages of doing so:firstly, the water in the spray has the effect of cooling the syngasproduced in the plasma unit due to promotion of the endothermic reactionof water with carbon (to produce hydrogen and carbon monoxide).Secondly, the overall chemical enthalpy of the produced syngas isincreased, allowing a greater export of electrical power if the gas isused to generate electricity. (ie giving an improvement in the overallnet electrical conversion efficiency).

The plasma treatment step will provide a secure disposal route forresidues produced by the process such a Flue gas cleaning residues.

The waste may contain constituents which contain hazardous compounds andelements, such as heavy metals, which are environmentally detrimental ifairborne. These may be termed APC (Air Pollution Control) residues andmay be present in the waste to be treated in an amount of .about.0.2% byweight. As these residues may be contaminated with heavy metals such aslead, zinc and cadmium, they will be classified as hazardous.Preferably, the process of the present invention further comprisesinclusion of hazardous inorganic materials, such as heavy metals andcompounds containing heavy metals, into the slag phase of the plasma.This will trap the hazardous materials in an inert non-leachable slag asan inert waste, thus providing a long-term solution for the disposalproblem for these materials.

The process may further comprise addition of one or more fluxing agentssuch as lime, alumina or silica sand to the plasma unit before or duringplasma treatment of the offgas and char. The advantage of adding afluxing agent is that in certain situations, it would ensure that a lowmelting point, low viscosity slag was produced from the inorganic,non-combustible materials. A fluxing agent such as silica sand, aluminaor lime may also be used to immobilize heavy metal species. Thesefluxing agents are preferably added to the char prior to introduction ofthe char to the plasma unit, and if the process is a continuous process,the additions may be made to the char stream.

The throughput and chemistry of the gas and char reactants entering theplasma unit are preferably maintained under steady state conditions.This should be achievable by the close control of the feed preparationsystem and primary gasifier upstream of the plasma unit.

The type, proportion and total addition rates of oxidant to the plasmaunit will be closely controlled and will take account of a number offactors:

the throughput and chemistry of both the char and gas reactants;

the knowledge that the addition of steam as an oxidant is effective inensuring rapid reaction rates with the pyrolysed solid char and sootproducts in the gas phase. It can help control the thermal stability ofthe plasma unit, avoiding the possibility of thermal “runaway”;

the addition of oxygen generates heat as a result of the exothermic(partial) combustion reactions that occur;

it is likely that steam will be used in combination with oxygen oroxygen enriched air for reasons of economy, efficacy of gasification ofthe char, destruction efficiency of the organics, quality and calorificvalue of the gas product and overall controllability of the process;

air may be used either in combination or as an alternative to oxygen.Although air is inexpensive to use, it is thermally less efficient thanoxygen, produces a much lower calorific gas product (due to the dilutioneffect of nitrogen) and may generate NOx as a by-product; and

the overall process economics, (which will be sensitive to localfactors).

If the chemical composition and mass throughput of the reactants aregenerally constant, then the ratio of oxidant to the reactant streams(containing the waste) will also preferably be maintained at a constantvalue. An increase in the feed rate of the reactants will preferablylead to a proportionate increase in the oxidant addition rate, which maybe controlled by automatic oxidant addition means. The electrical powersupplied to the plasma will also preferably be adjusted to match thechange in the feed rate of the waste to the plasma unit and will takeaccount of the thermo-chemistry of the system and the thermal lossesfrom the unit.

The gas exiting the plasma unit may be maintained at a temperaturegreater than 1000° C., preferably between 1000° C. and 1500° C. mostpreferably between 1000° C. and 1300° C. Excessive off-gas temperatures(ie >1300° C.) are not desirable as this increases the plasma powerheating requirement, reducing the net export of electricity from theplant.

Preferably, the gas produced from the gas plasma treatment is used in aturbine or gas engine to generate electricity. The turbine may be aconventional boiler steam turbine or gas turbine. The syngas resultingfrom the plasma treatment process is preferably cooled or allowed tocool to a temperature to below 200° C. prior to use in a turbine. Thisallows the partially combusted components of the gas, e.g. carbonmonoxide, to be combusted completely and efficiently. Additionally, ifthe syngas from the plasma treatment is cooled using a heat exchangesystem which transfers the heat to another (heat transfer) gas,preferably the heat transfer gas is used to heat a steam turbine foradditional power generation.

The plasma unit preferably comprises a stainless or carbon steel weldedshell lined with high grade refractory lined bricks.

Preferably, the plasma unit comprises remote water cooled copperelements, which will preferably be employed to effectively contain themolten inorganic phase(s). These elements preferably act to form aprotective frozen melt layer on the hot face refractories to promotegood refractory performance.

Preferably, the gasifier comprises an exhaust gas port in fluidconnection with the plasma unit. Preferably, the exhaust gas port in thegasifier will be closely coupled to the plasma unit to preventcondensation of tar or volatile salts in the channel connecting the twounits.

Preferably, the plasma unit comprises either a single or twin graphiteelectrode system to generate the plasma arc. Three possibleconfigurations and the method by which they are interconnected to theplasma power supply are shown in FIG. 1. Each of diagrams (a) to (c)shows a schematic drawing of a furnace having two electrodes. The‘molten bath’ refers to the molten slag present at the bottom of thefurnace.

In diagram (a), an electrode is located in the roof of the furnace andanother electrode is located at the base of the furnace. Both electrodesare connected to a power source to enable generation of plasma withinthe furnace.

In diagram (b), the same configuration as in diagram (a) is shown, withan additional start electrode (shown to the left of the furnace) toenable ease of start-up of the plasma generation system, as would beappreciated by the skilled person.

In diagram (c), two connected electrodes are located in the roof of theplasma unit.

Preferably, one or more electrode(s) will be located in the roof of theplasma unit. The plasma unit may preferably comprise water cooledelectrode seals at the inlets and outlets of the unit.

Preferably, the graphite electrode(s) will be drilled, and a plasmastabilizing gas (eg nitrogen or argon) will be injected down the centreof the electrode(s).

Optionally, the electrodes are coated with a refractory material (egalumina coating) in order to reduce the wear of the electrode.

Optionally one or more water-cooled plasma torches may be used togenerate the plasma.

The plasma unit may comprise one or more feed ports for the introductionof the char residue from the gasification process. Preferably, the charresidue is introduced into the plasma unit via one or more feed ports inthe roof of the unit. The feed ports will preferably be located remotelyfrom the slag removal spout.

The plasma unit may comprise one or more gas entry feed ports for theintroduction of the offgas into the plasma unit; the feed ports may belocated in a sidewall or the roof of the plasma unit. The tar-laden gas(the offgas) from the gasifier will preferably enter the plasma uniteither through a port in the sidewall or roof. Preferably, the plasmaunit will be designed to prevent or minimise short circuiting of thedirty gas, for example:

preferably, the point of exit for the reformed gas (the syngas) will bediametrically opposed and as remote as practical to the point of entryof the gases and/or

the offgas will preferably be forced downwards in the plasma unit (egeither by flow direction devices or else by locating the exhaust port ata lower level than the gas entry point thereby reducing the buoyancyeffect of the gases.)

The plasma unit will be designed to ensure adequate residence time forboth the char and gas reformation reactions to occur.

The oxidant may be injected into the plasma unit to enable thegasification of the carbon component of the char and reformation of thedirty, tar-laden gas stream (the offgas) from the gasifier unit.

The oxidant injection point will preferably be remote from theelectrodes to prevent high graphite wear rates.

The plasma unit may comprise separate and multiple points of injectionfor the oxidant, ideally at least one for injection point for the offgasand at least one injection point for the char residue. Alternatively,the char and offgas may be introduced through a single point ofinjection.

An injection means may be provided in the plasma unit for the injectionof the oxidant and the injection means is preferably such that wheninjected a radial flow of oxidant will result. This would improve thecontacting between the oxidant and reacting “fuel” phases (i.e. theoffgas and the char).

The char may contain an inorganic fraction, i.e. solid componentscontaining elements other than carbon. The inorganic fraction of thechar will form a molten complex oxide “slag” phase that, preferably,will be continuously removed from the plasma unit. The unit maytherefore comprise a means for removing the slag phase, which may be inthe form of a slag overflow spout angled upward (toward the exterior ofthe unit), so the molten slag exiting the plasma unit will create anairlock to prevent either air ingress or gas egress from the unit.

During use, the plasma unit will preferably be tightly sealed. The unitwill preferably be maintained under positive pressure.

Preferably, a gas tight, bolted flange will be used to seal the roof tothe main furnace body section. Preferably, the flanged bolts will bespring-loaded to ensure that in the unlikely event of high overpressurein the plasma unit, (eg as a result of an explosion) the roof will beraised to allow rapid dissipation of pressure. The escaping gases willbe safely handled via a fugitive emissions handling system.

The presence of carbon soot or other conductive deposits in the unit mayencourage the generation of side-arcs (also referred to as parasiticarcs) which emanate from the electrode(s) and transfer to the roof orthe sidewalls of the unit rather than to the melt. Side-arcs tend to bedestructive, leading to premature failure of the reactor shell. A numberof measures may be in place to prevent side-arc development fromoccurring:

preferably, the roof of the plasma unit will be constructed in sectionswhich will be electrically isolated from each other.

Close attention will be paid to the design of the electrode seal toavoid the possibility of electrical tracking to the roof. All holdingbolts, securing the seal will preferably be electrically isolated and,preferably, dust protected to avoid build-up of dust on electricallyconductive surfaces.

Gas purging will preferably be employed around the outside of theelectrode(s) to prevent the build-up of deposits on surfaces that are inclose proximity to the electrode. The unit is preferably adapted in away that will minimize the production of soot or tarry products.

All seals will be designed to be easy to clean and/or replace ifrequired.

The off-gas composition will preferably be continuously monitored and afeed back control loop may be employed to adjust the power and oxidantfeed rate to the plasma unit.

The reformed gas (syngas), which results from the plasma treatment, willpreferably be further cleaned to remove acid gases, particulates andheavy metals from the gas stream to produce a fuel that can be use inthe generation of electricity and heat for steam raising.

Optionally, the apparatus may further comprise a pyrolysing unit.

The process may further comprise collecting the gas produced in theplasma treatment unit (commonly called a syngas).

Typically, the plasma treatment unit will generate a solid and/or moltenmaterial, as would be know to the skilled person. The process mayfurther comprise collecting the solid and/or molten material produced inthe plasma treatment unit.

The apparatus may further comprise a unit for the aerobic microbialdigestion of waste which may be as described herein.

As mentioned above, the process preferably further comprises subjectingthe waste to microbial digestion, more preferably aerobic microbialdigestion, prior to the gasification step. This has the added advantagesof producing a more homogenous feedstock with a higher calorific contentand less moisture content than unprocessed waste, which allows for amuch more efficient combined gasification and plasma process. Thegasification process is far more efficient with a feedstock ofrelatively consistent calorific value. Likewise, it has been found thatan efficient plasma treatment should ideally have a relativelyhomogenous feed of offgas. By treating the waste initially with amicrobial treatment to homogenise the waste introduced to the gasifier,the resultant offgas from the gasifier is also more consistent incalorific value and hence the process as a whole is more efficient.

Preferably, the aerobic microbial digestion is carried out in a rotaryaerobic digestion unit.

Preferably, the waste is rotated in the rotary aerobic digestion unit ata rate of from one revolution every minute to one revolution every tenminutes.

The moisture content of the waste prior to aerobic digestion may be from20 to 75% by weight, preferably 25 to 50% by weight.

Preferably, the waste has an average moisture level of 45% or less,preferably 30% or less, after the aerobic digestion treatment.

The microbial digestion step preferably comprises the steps of:

mixing a (first) supply of waste having a first average moisture levelbefore treatment with a supply of other waste, having a lower averagemoisture level before treatment, wherein the relative quantities byweight of the first waste and the other waste are controlled, feedingthe mixed waste into a microbial treatment vessel, treating the waste bymicrobial activity in the treatment vessel, the mixed waste beingagitated during treatment, the oxygen content in the gas in contact withthe mixed waste being controlled during the treatment process so that itdoes not fall below 5% by volume, the mixed waste having an averagemoisture level after treatment not exceeding 45% by weight, morepreferably not exceeding 35% of weight and most preferably not exceeding25%.

Subsequent drying of the product to an average moisture content of below20% by weight can be carried out relatively easily. Preferably, thefirst supply of waste comprises organic waste, preferably solid organicwaste. The other waste may comprise solid waste.

The part of the apparatus of the present invention for carrying out themicrobial digestion preferably comprises:

a supply for a first waste having a first average moisture level beforetreatment and a supply for other waste having a lower average moisturelevel before treatment,

means for mixing the first waste and the other waste,

control means for controlling the relative quantities by weight of thefirst waste and other waste mixed together,

means for feeding the first waste and the other waste to a treatmentvessel,

means for agitating the solid organic waste in the treatment vessel,

drying means following the treatment vessel and

means for controlling the air flow through the treatment vessel, and/orthe input of first waste and other solid waste to the treatment vessel,so that the average moisture level of waste after treatment does notexceed 45% by weight, more preferably not exceeding 35% by weight andmost preferably not exceeding 35% by weight, and so that the oxygencontent of gas in contact with the mixed waste in the vessel does notfall below 5% by volume.

Variations in the physical composition (for example calorific content)and moisture level of the first waste (typically domestic waste, butalso possibly agricultural waste) can be ‘smoothed out’, so that aproduct formed from treated waste from different areas or different timeperiods can be relatively homogeneous.

The waste, either the first and/or the other waste, treated using themicrobial step is preferably “organic waste”, preferably solid organicwaste, for example domestic waste, industrial waste or agriculturalwaste. “Organic waste” is waste that has at least a proportion oforganic material capable of being treated microbially. The other wastemixed with the first waste preferably also contains organic material.

By, “mixing” it is meant that at least two separate sources of waste arecollected and fed into the microbial treatment vessel in controlledrelative quantities by weight. The waste from the two different sourcesmay be mixed in a mixing device or in a shredder or they may be mixedduring agitation in the treatment vessel.

The microbial digestion step will preferably produce heat. Thisbreakdown is accelerated by changes in the physical nature of the waste.Typically, the microbial activity is bacterial activity. Preferably, themicrobial activity is aerobic.

The microbial digestion process is preferably carried out using bacteriain the thermophilic phase, which normally occurs in the temperaturerange 60° C.-75° C., most preferably around 63° C.-70° C. In this phase,very rapid digestion occurs with the production of heat. It is foundthat the reaction in the thermophilic phase is much quicker than thecommonly used mesophilic phase which occurs in the range 30° C.-38° C.

Accordingly, accelerated decomposition of the waste takes place.However, if the temperature rises above 75° C., there is a danger thatthe bacteria will be destroyed.

The microbial reaction in the thermophilic phase results in the naturalgeneration of heat which breaks down the waste to produce a materialwhich is suitable for processing to provide a fuel or compost. Themicrobial reaction will almost always provide sufficient heat tomaintain itself without provision of supplementary heat. However, inpractice, chemical mixing of the waste can lead to an increase intemperature which assists the commencement of the microbial activity.

Other material may be added to the microbial treatment vessel, forexample quicklime, to control pH.

Preferably the oxygen level in the gas which is in contact with thewaste being treated in the microbial digestion step does not fall below5% by volume.

The treatment vessel for carrying out the microbial digestion is notnormally filled completely, so there is a gas space above the wastebeing treated. The oxygen content in this gas space is suitably measuredand preferably controlled. The skilled person will be aware of suitabletechniques for measuring and controlling oxygen content. The moisturelevel may also be measured, as described below.

Preferably, the oxygen content (and, optionally moisture level) of gasremoved from the treatment vessel (as will be described further below)is measured. This is a particularly convenient arrangement.

The gas in the microbial treatment vessel will typically compriseatmospheric nitrogen, oxygen, carbon dioxide and water vapour. This gasmay contain no methane, ammonia or hydrogen sulphide, as the microbialactivity is carried out in the thermophilic phase.

In order to maintain the oxygen level above 5% by volume, air or oxygencan be supplied to the treatment vessel. Air or oxygen can be suppliedcontinuously throughout at least part of the process or in discreteinputs of air/oxygen.

In order to replace the oxygen which promotes aerobic digestion and tocontrol moisture level in the exit gas, (the gas exiting the microbialtreatment vessel) a relatively high airflow rate is required.

The air can be supplied by some form of forced draught. For example, afan may be provided. The fan may blow air into the microbial treatmentvessel. However, it is preferred that there is a fan to draw gas out ofthe microbial treatment vessel. Where extraction means are provided forwithdrawing gas from the microbial treatment vessel, it may be replacedby air supplied through at least one duct. Air can be supplied to themicrobial treatment vessel intermittently, but it is preferably suppliedsubstantially continuously. The microbial treatment vessel may not besubstantially sealed, so that as long as gas is removed, air willnaturally flow in through openings to replace the gas removed.

As fresh air is supplied to the microbial treatment vessel and as gas isremoved from this vessel, water vapour will be removed from the waste.This helps to control the drying effect, leading to a product having anaverage moisture level within the desired range.

Air supplied to the microbial treatment vessel may be previously driedby any suitable apparatus, to maximise the drying effect.

According to a preferred aspect of the invention, the moisture level inthe gas in contact with the waste in the microbial treatment vessel ismaintained at a level below its dew point. This ensures that water issubstantially continuously removed from the waste being treated into thegas space by evaporation.

Means may be provided in the microbial treatment vessel for monitoringthe moisture level in the gas space. Any suitable means may be employedfor measuring the moisture level.

The moisture level in the microbial treatment vessel may be maintainedbelow the dew point by supplying air which has a moisture level belowthe dew point of the waste being treated at the temperature oftreatment. As the temperature of the microbial digestion will betypically higher than ambient temperature, normal fresh air may be used.Alternatively, dried air, having a moisture level below the moisturelevel of ambient air, may be used. The main process features whichmaintain the oxygen level within the required range can also be used tomaintain the moisture level within the required range.

The flow of air and gas through the microbial treatment vessel alsoremoves heat from this part of the apparatus. It is found that anadequate heat balance can be achieved. That is, heat generation by themicrobial activity within the concentrated mass of waste can be balancedwith heat removal by the gas flowing through the vessel so that thetemperature is maintained at a desirable level.

Preferably, the waste should be agitated during the microbial digestion.This provides further breakdown of the waste and mixing to ensure thatmicrobes are spread throughout the material. It also exposes-differentparts of the waste to the gas to ensure access of oxygen to the wasteand drying of the waste by the gas. Agitation may take place by anysuitable means, but it is particularly preferred that the digestiontakes place in a rotary aerobic digestion unit, i.e. a unit containing arotating aerobic drum.

The drum may be rotated at any suitable rate, and suitably completes onerevolution in a time range of 1 minute to 10 minutes, preferably 2-5minutes, most preferably about 3 minutes. However, a higher rate ofrotation may be used during loading and unloading of waste into/out ofthe microbial digestion unit, in order to assist these operations.Typically, the speed can be increased to one revolution per minuteduring loading and unloading.

As will be described further below, the drum is suitably simultaneouslyloaded with waste at one end and unloaded with microbially treated wasteat its other end. Loading and unloading typically take place at 4 hourlyintervals and can take 30 minutes.

The drum preferably comprises a substantially parallel sided circularsection cylinder. The axis of the cylinder may be inclined to thehorizontal, for example at an angle in the range 30-10.degree. mostpreferably 5.degree.-8.degree., to provide gravitational flow throughthe drum.

Any suitable size of drum may be provided, depending upon the rate ofconsumption of waste. It has been found that, for a processing rate ofabout 250-500 tonnes per day, a drum of diameter in the range 3.5-6 m,preferably 4-6 m most preferably around 5.5 m should be used. The lengthshould be in the range from 6 to 10 times the diameter, most preferablyabout 8 times the diameter, suitably up to 40 m.

The drum may be used of any suitable material, for example mild steel.

A rotary drum has the advantage that it is mechanically simple. Thereare relatively few problems of blocking and very few moving parts, whichreduces the risk of breakdown.

The agitation caused by the rotation leads to attrition of the waste,further contributing to its breakdown. Preferably, the drum is filled toa high level with waste, being preferably initially 75% to 90% full byvolume. This leads to increased attrition, rapid heat generation andalso to efficient use of microbial treatment vessel.

Average residence time of the waste in the microbial treatment vessel issuitably in the range 18-60 hours, more preferably around 24 to 48hours, most preferably around 36 hours.

The microbial treatment vessel preferably comprises a vessel throughwhich the waste is moved during treatment, for example a drum asdescribed above. The waste suitably moves from a loading point to anunloading point within the drum. As noted above, loading and unloadingsuitably occur substantially simultaneously, with fresh (microbiallyuntreated) waste being loaded at the loading end and mixed solid treatedwaste being removed at the unloading end. The loading and/or unloadingoperation can take 10-40 minutes, preferably about 30 minutes.

One unloading operation or loading operation is preferably spaced fromthe following unloading or loading operation respectively by a period inthe range 2-8 hours, preferably 3-5 hours, most preferably around 4hours. In this way, a “semi batch” process can be carried out.

During processing, it is found that the volume of the material maydecrease by as much as 25%. The gas space over the material willaccordingly increase.

The waste material should be discharged from the treatment vessel at astage at which the treated waste material is sufficiently digested andsufficiently dry. This typically occurs after a period of about 48hours. By restricting residence time to 48 hours or less, additionalloss of carbon can be reduced.

It has been found that microbial treatment is effective in reducing thesize of some constituents of the waste. Nevertheless, further processesto assist size-reduction of the waste constituents may be used. Forinstance, in order to promote the microbial activity, some parameters ofthe waste fed to the digestion step are preferably controlled. Forexample, the waste is preferably treated in a first process before thedigestion step (or the gasification step, if the process does notinclude a microbial treatment step) to remove particles of size inexcess of 100 mm, preferably 60 mm, more preferably 50 mm. This firstprocess may comprise a first step in which very large objects areremoved, for example by hand or by sieving and a second step in whichthe remaining material is treated to reduce its particle size, forexample by shredding. The person skilled in the art will be able toobtain suitable shredding apparatus. Shredders can either have one fixedrotor or two counter-rotating rotors.

Alternatively, (prior to the microbial or gasification step), the wastemay be subjected to an operation to reduce its particle size, forexample by shredding without initially removing oversized particles. Theshredding operation is particularly beneficial for the microbialtreatment process, as it mixes the material thoroughly, spreading themicrobial culture throughout the material and initiates a thermophilicreaction very quickly. Shredding may be used to reduce the spacingbetween the particles to promote the microbial reaction.

The second parameter which may be controlled is the average moisturecontent of at least some of the waste treated in the microbial treatmentstep. The average moisture level of this part of the waste is suitablyin the range 20-75%, more preferably 30 to 60%, most preferably 30 to50%.

All moisture levels quoted herein are % by weight. They are averagevalues, being averaged for quantities of at least 100 kg of waste.

Moisture levels of waste may be measured by measuring the moisture levelof air or gas over the waste at a fixed temperature and in equilibriumwith it.

If the waste after mixing is low in organic content or moisture level,process water maybe preferably added in controlled quantities. Thisprocess water is preferably waste water from water treatment, mostpreferably dewatered sewage sludge. This material has a high nitrogencontent and acts as a catalyst for the microbial reaction.

As mentioned above, a desirable moisture level of the waste treated inthe microbial treatment step may be obtained by blending a first wastewith other waste of a lower average moisture level. It is found thatmixed domestic waste typically has a moisture level in excess of 30% byweight. Commercial waste from offices and factories is typically drier,having a moisture level in the range 10%-30% by weight.

The moisture level of waste fed to the digester may be manipulated byaltering the mixing ratios of different types of waste. Preferably atleast part of the waste fed to the microbial digester has a moisturelevel in the range 20-75% by weight, preferably 25 to 65% by weight inorder to promote the faster thermophilic reaction. However, part of thewaste fed to the digester may comprise a relatively dry commercialwaste. The heat generated by the digestion of the moist waste issufficient to treat the whole of the waste fed to the treatment vessel.However, during the agitation process, the commercial and domestic wasteare slowly mixed together reducing the overall moisture content of themixture, so that at the end of the processing, the moisture level doesnot exceed 45% by weight and preferably does not exceed 25% by weight.

The first waste with higher moisture level may be blended with otherwaste with lower moisture level in blending apparatus in a controlledmanner. The relative quantities of different types of waste arecontrolled so that the desired average moisture level over the combinedmasses of mixed wastes is obtained as explained above.

The blending step also allows absorbent material such as paper and paperbased material (which is particularly common in commercial waste) to beblended intimately with the moist waste (such as domestic waste). Theabsorbent material absorbs liquid rich in bacteria, providing asubstrate for the bacteria to grow on and allowing the bacteria to bespread throughout the waste being processed. This promotes reaction andmixing, leading to an improved digestion. Further, the wetting of thepaper helps it to be broken down.

In processing the waste in the microbial treatment step, it is desirableto produce a product which is substantially homogeneous, such that itsconstituents are particles have a relatively small size distribution,the particles have a largest measurement of 50 mm or below.

The blending step helps to improve the homogeneity of the product.

However, although blending takes place, it is found that the moisturelevel remains concentrated in local areas of the waste, where it issufficiently high to allow the thermophilic reaction to commence andproceed very rapidly.

The relative quantities of different types of waste feed can becontrolled using automatic weigh feeders.

By way of example, the moisture level of the waste during the microbialtreatment may be as follows:

Domestic waste with a high organic content and moisture level above 50%can be mixed with commercial waste having a moisture level of 20% orbelow in a suitable ratio to provides a blend having an average moisturelevel in the range 45 to 55% by weight.

During microbial digestion, a part of the moisture is absorbed by thegas and air flowing over the material being processed. The averagemoisture level may drop to around 30-40% by weight, preferably 25 to 30%by weight.

During emptying of the microbial treatment vessel, the waste which stillhas a high residual heat level, may be dried by a forced draught asdescribed above, so that the moisture level drops to the range 30-40% byweight, preferably 25 to 30% by weight.

The waste treated in the microbial digestion step may then be furtherdried on a drying floor as described above, so that the moisture leveldrops to below 25% by weight.

A further parameter which may be manipulated is the pH of the waste inthe microbial treatment process. This pH of the waste in the microbialtreatment process is preferably of from 6.0 to 8.5, preferably 6.3 to7.3, most preferably around 6.8.

Nitrogen level has an impact on microbial activity, and adjustment of pHand nitrogen content can be advantageous.

It has been further found that the density of the waste fed to themicrobial treatment vessel is suitably not too low. Preferably, thedensity is not less than 450 g per litre, preferably not less than 750 gper litre. Again, the blending step is particularly useful here.Domestic waste can have a relatively high density. The average densitycan be controlled by admixing a suitable quantity of commercial waste,which has a comparatively low density.

Preliminary Treatment

As described above, the waste may be subjected to various types oftreatment before the gasification or microbial digestion step (‘previoussteps’). Preferably, the previous steps include any or all of thefollowing:

1. Picking

Initial treatment to remove objects which are not readily combustible,such as stone, concrete, metal, old tyres etc. Objects having a size inexcess of 100 mm or more may also be removed. The process can be carriedout on a stationary surface, such as a picking floor. Alternatively oradditionally, the waste may be loaded onto a moving surface such as aconveyor and passed through a picking station in which mechanical ormanual picking of the material takes place.

2. Shredding

Shredding is a highly preferred step. It is carried out to reduce theaverage particle size. It can also be used to increase blending of wastefrom different sources. It also makes the treatment process moreeffective. It is found that, during the shredding process, microbialactivity may commence and rapidly raise the temperature passing veryquickly through the mesophilic phase into the thermophilic phase.

3. Screening

The waste may be mechanically screened to select particles with size ina given range. The given range may be from 10 mm to 50 mm. Material lessthan 10 mm in size comprises dust, dirt and stones and is rejected. Thewaste may be treated to at least two screening processes in succession,each removing progressively smaller fractions of particles. Materialremoved in the screening process as being too large may be shredded toreduce its average size. Material which is classified by the screen asbeing of acceptable size and, where applicable, shredded material canthen be fed to the treatment vessel.

Subsequent Treatment

The waste may be subjected to a number of steps after the microbialdigestion treatment step and before the gasification step. These stepsmay include any of the following:

1. Grading

The material may be screened to remove particles in excess of a givensize. For example, particles in excess of 50 mm may be rejected. Theymay be subsequently shredded to reduce their size, returned to theaerobic digester or simply rejected.

2. Metal Separation

Relatively small metal particles such as iron or aluminium may havepassed through the system. They can be removed, for example by amagnetic or electromagnetic remover in a subsequent step. Metalparticles removed from the system may then pass to a suitable recyclingprocess.

3. Drying

Suitably, after treatment in the microbial treatment vessel, the wasteis subjected to an additional drying step. If the moisture level doesnot exceed 45% by weight, more preferably does not exceed 35% by weightand most preferably does not exceed 25% by weight, after the microbialtreatment, the subsequent drying can be carried out relatively simply.For example, in a first drying stage, a forced draught of air may beprovided during or after the unloading phase from the treatment vessel.During this stage, the waste treated by the microbial digestion stagewill still be at high temperature (for example in the range 50-60° C.)and further moisture can be removed simply by forcing air over it. Afurther drying step may comprise laying the material out on a dryingfloor. In this step, waste is laid out at a thickness of not more than20 cm over a relatively large area for a suitable period of time, duringwhich the moisture level drops. The waste may be agitated, for exampleby turning using mechanical or manual apparatus such as a power shovel.The waste may be turned at intervals of for example of 2-4 hourspreferably around 3 hours. Preferably, during this stage, the moisturelevel drops to below 25% by weight after which no further biologicaldecomposition occurs. Suitably, the waste is left on a drying floor fora period in the range 18-48 hours, preferably 24-36 hours, morepreferably around 24 hours. It is also found that further drying maytake place during subsequent processing, due to the mechanical input ofenergy. Waste heat from other process equipment, for example from thegasification and/or the plasma treatment step, may be used to dry thematerial. Air warmed by the heat generated in the gasification and/orplasma treatment steps may be blown into the microbial waste treatmentvessel and over or through the waste to increase the drying rate ofthese processes.

Alternatively, the drying apparatus may comprise a rotary flash drier orother drying device.

4. Pelletising

In order to convert the treated waste to fuel, the waste may beclassified according to size and subsequently densified to providepellets of suitable size for use in the gasification step. During thispelletisation stage, further drying of the waste may occur, due to heatgeneration caused by friction and due to further exposure to air.Preferably, in order for pelletising to proceed well, the moisture levelof the treated material is in the range 10-25% by weight.

It has been found that the microbial treatment step can be adapted toprovide a fuel for use in the gasification step, referred to as GreenCoal, which has a calorific value in the order of 14.5 MJ/kg which isabout half that of industrial coal.

By blending different sources of waste material, fuel produced by themicrobial treatment step at different times or with waste from differentlocations can be relatively homogeneous in terms of:

1. Calorific value—suitably in the range 13 to 16.5 MJ/kg, preferably12-15 MJ/kg. The calorific value may be higher if the contents have beensignificantly dried.

2. Density—suitably in the range 270-350 kg/m.sup.3 more preferablyaround 300 kg/m.sup.3.

3. Moisture level—below 30% by weight and preferably around 20% byweight.

The process of the present invention may comprise a pyrolysis step priorto the gasification step, and after the microbial digestion step, ifused. The waste that results from the microbial digestion step may beused to supply a feed to a pyrolysis process, as described below.

The apparatus of the present invention may include means for feedingmicrobially treated waste from the treatment vessel to a means forpyrolysing the treated waste (i.e. a pyrolysis unit).

If the process involves a pyrolysis step prior to the gasification step,preferably the pyrolysed waste is fed to the gasification unit, wherethe gasification takes place. This will normally require the pyrolysedmaterial to be at a high temperature and the gasification processpreferably occurs directly after the pyrolysis process.

The apparatus may comprises a microbial digestion unit in fluidconnection with the gasification unit, and the gasification unit may bein fluid connection with the plasma treatment unit, to allow wastetreated from the microbial treatment to be transported to thegasification unit, and to allow offgas and char resulting from thegasification step to be transported to the plasma treatment unit.

The apparatus may be adapted to treat the waste in a continuous process.Microbial digestion step may be typically be carried out in a semibatch-wise fashion, whereas the pyrolysis and gasification processestypically require a continuous feed of material, an interim storagemeans, for example in the form of a feed hopper may be provided. It ispreferred that there is a first delivery means for receiving treatedwaste from the microbial treatment process and feeding it into theinterim storage means and a second feed apparatus for feeding the storedtreated waste from the interim storage means to the pyrolysis apparatusor the gasification apparatus. The second feed means is preferablyoperated substantially continuously. The first and second feed apparatusmay comprise any suitable means, for example conveyor belts or screwfeeders.

A preferred embodiment of the process of the present invention isillustrated in FIG. 2, which shows:

a first step in which the raw waste is subjected to aerobic microbialdigestion in a rotary aerobic digestion unit (RAD),

a second step comprising gasifying the products of the rotary digestionstep in a gasifying unit (gasifier), which produces an offgas and achar,

a third step comprising treating the char and the offgas to a plasmatreatment process in a plasma unit (plasma furnace), producing avitrified solid slag (which is discarded) and a syn gas,

a fourth step comprising cleaning the syngas,

a fifth step comprising either exhausting the syngas or combusting thesyngas in a gas engine or gas turbine (termed a ‘power island’ in theFigure) to produce electrical energy, and then exhausting the combustedsyngas. The heat produced in combusting the syngas or in the plasma stepmay be used to dry waste material (not shown).

A further preferred embodiment of the process of the present inventionis illustrated in FIG. 5, which shows:

Step A, in which the raw waste is subjected to aerobic microbialdigestion in a rotary aerobic digestion unit (RAD),

Step B, in which the waste feedstock resulting from Step A is treated ina gasifier, to produce an offgas and a char, both of which are thentreated in a plasma unit at 1500° C.,

Step C, in which the hot gases produced in Steps B and/or I are cooledin a gas cooling system,

Step D, which comprises optionally treating the gas to a cleaning step,

Step E, which comprises optionally compressing and storing the gas,

Step F, in which the gas from step E is passed through a gas turbine,which is directly coupled to a generator (EG2—not shown) to generateelectricity,

Step G, in which the gas is passed through a heat recovery steamgenerator,

Step H, which involves exhausting the gas to a stack and monitoring theflue gas,

Step I, in which high pressure steam from step C and/or G is passedthrough a steam turbine to generate electricity with electricitygenerator 1 (EG1). The low pressure steam from the turbine is passed viaa close-coupled condenser to a separate cooling tower in Step J and to afeed-water system in step K. The electricity generated in Step I and/orF may be distributed in Step L either to any part of the apparatus(represented by step M) or transferred externally (step N).

As indicated above oxygen and/or steam may be introduced to thegasification unit or pyrolysis unit and/or plasma treatment unit.

The present invention will now be further exemplified in the followingnon-limited Examples

EXAMPLES Gasifier Set-Up and Operation (see FIG. 3)

The FBG (fluid bed gasifier), comprises a vertical, cylindrical, mildsteel vessel lined with a composite refractory lining. The externaldimensions of the gasifier shell are 1.83 m diameter.times.5.18 m highand the internal diameter is 0.254 m; the height of the expanded bed isapproximately 1.0 m.

The FBG uses a heated bed of alumina silicate ceramic particulates asthe bed media. RDF (refused derived fuel) feedstock is fed continuously,at a controlled rate, to the FBG 1 through a solid fuel feeder system.The as-received feed is transferred by a belt conveyor 2 to a surgehopper 3 where a variable-speed screw conveyor controls the volumetricfeed rate of the solids. These discharge into an airlock. A constantspeed screw conveyor is employed to transfer the feed from the airlockto the fluidised bed 1 where it is charged above the upper surface ofthe bed. Additional inert gas purging is used at the hopper and at theairlock to prevent air ingress or gas egress into the feed stream.

A propane fuelled under-bed preheating system is used to raise thetemperature of the bed to 420° C. At this point wood pellets are fedthrough a separate feeder into the airlock to raise the temperature ofthe bed to 600° C. when the secondary propane supply is discontinued,then at 700° C. the primary propane supply is turned off. The woodpellet feed is continued to attain the operating temperature of 800-850°C. when it is replaced by RDF.

Oxygen is supplied from a ‘Titan’ multi-pack of 10-11 cylinders. Theflow rate is controlled through a mass flow controller (MFC) rated up to500 Nlpm.

The oxidants: oxygen and steam, are mixed prior to injection through anupward facing nozzle located below the bed. The individual steam andoxygen feed rates are closely metered to match the feed rate of the RDFto ensure that the gasifier operates within the design operating limits.

Multiple pressure and temperature sensors are used to closely monitorand control the FBG operation. Safety interlocks are built in to ensuresafe shut-down or alarming of the system in the event of the unitfalling outside the specified operational limits.

The offgas exiting the FBG is transferred to the plasma converter unit 4in a refractory lined steel duct 5.

Plasma Converter Set-Up

A schematic drawing of the plasma converter (excluding the electrode andmanipulator arrangement) is given in FIG. 4 and comprises the followingsections:

i) A refractory-lined mild steel shell 6 with an additional doubleskinned water cooling jacket in the upper shell section and a series ofwater cooled copper fingers 7 which provide additional protection forthe refractories at the slag line. The refractory is a cast aluminaspinel containing 91% Al.sub.2O.sub.3, 7% MgO and 2% CaO with a maximumservice limit of 1800° C. A 150 mm diameter cylindrical steel bar in thebase of the converter provides the return (anode) electrode for singleelectrode operation. A taphole 8 in the furnace hearth allows forintermittent removal of the molten slag. The converter has apertures inthe upper shell region for pressure monitoring and for camera viewing.The refractory temperatures are monitored at eight locations usingB-type thermocouples (up to 1800° C.) and in the return electrode at twolocations using K-type thermocouples (up to 1300° C.).

ii) A conical mild steel, refractory-lined, water jacketed roof 9 withfive large apertures: a central port for single electrode work 10, aside port for gas feed from the FBG feed 11 a off-gas port 12 and a portfor solids feeding of the oversized bed material (not shown) and a sparegeneral access port 13. There is also a smaller camera port housing asmall remote video camera in a protective case which allows for viewingof the inside of the plasma converter. There are two thermocouple holesfor refractory temperature monitoring as above. The roof also provideslocation points for electrode manipulators and for the off gas ducting.

iii) A steel support stand 14, mounted on heavy-duty wheels and railwaytracks for easy removal and installation of the plasma converter.

iv) Electrode 15 and Manipulator System 16 where the cathode electrodemotion is controlled by a central, single axis manipulator (verticalonly), consisting of a heavy-duty linear slideway actuated via aservo-motor and gearbox. The electrode clamping device 17 is fixed tothe carriage plate and the whole assembly is mounted on electricallyinsulating ceramic and fibreglass rings and spacers to prevent sidearcing of the plasma device. The base of the manipulator is surmountedby a seal assembly containing a water cooled packing gland type seal forthe torch or electrode to pass through into the plasma converter.Electrode diameters of up to 100 mm can be accommodated through thiscentral port and the maximum stroke is 1000 mm. The graphite electrodeis centrally drilled and inert plasma gas is injected through thisconduit.

The use of the single manipulator allows for single electrode (cathode)operating mode and the return path for the current is via a steel returnelectrode in the base of the converter (anode).

In operation, the dirty offgas from the gasifer flows via a refractorylined duct to the plasma converter. Additional oxygen and steam isaxially injected into the gas stream at the point of entry into theconverter.

The high temperature and addition of oxidants at the converter stagepromotes the cracking and reforming of organic species and gasificationof sooty and char products. The power to the plasma arc is controlled tomaintain a temperature of gases exiting the unit to .about.1000-1300° C.Ash particulates that are carried over from the gasifier will drop outand be assimilated in the melt. After treatment in the converter unitthe syngas exits via a second gas port in the base of the unit.

Example 1 Treatment of Biomass Wood Pellet

The general methodology for treating the wood pellet is as given above.The rate of feed of wood pellets to the gasifier averaged 42 kg/h. Asummary of the operating conditions employed in the FBG to maintain thebed temperature at around 800° C. and at the plasma converter to give an(estimated) exhaust temperature of 1250° C. is given in table 4. Thesefigures are in close correlation to the theoretically derived operatingrequirements.

TABLE 4 Example operating conditions for the Treatment of biomass (woodpellet) Plasma Item Gasfier Converter RDF Feed Input (kg/h) 42 PowerInput (kW) 79 Argon (1pm) 50 130 Oxygen (Nlpm) 189 61 Steam (kg/h) 14 0

Example 2 Treatment of RDF Material

The methodology for the treatment of the RDF material is as given above.The RDF was obtained from a commercial thermal treatment plant. Thegeneral composition of this material is given in table 1 above. Thematerials was fed at an average rate of 40.5 kg/h to the gasifier. Asummary of the operating conditions employed in the FBG to maintain thebed temperature at around 800° C. and at the plasma converter to give an(estimated) exhaust temperature of 1250° C. is given in table 5. It wasobserved that there was again good correlation between the theoreticaland experimentally derived values.

TABLE 5 Example operating conditions for the treatment of refuse derivedfuel Plasma Item Gasfier Converter RDF Feed Input (kg/h) 40.5 — PowerInput (kW) — 70 Argon (1pm) 50 130 Oxygen (Mlpm) 189 61 Steam (kg/h) 140

1. A process for the treatment of waste, the process comprising: (i) apyrolysis step comprising treating the waste in a pyrolysis unit toproduce an offgas and a non-airborne, solid char material; and (ii) aplasma treatment step comprising subjecting the offgas and thenon-airborne, solid char material to a plasma treatment in a plasmatreatment unit in the presence of oxygen and, optionally, steam, whereinthe plasma treatment unit is separate from the pyrolysis unit.
 2. Aprocess as claimed in claim 1, wherein the process further comprisessubjecting the waste to a microbial digestion step that is preferably anaerobic microbial digestion step, prior to the pyrolysis step.
 3. Aprocess as claimed in claim 1, wherein the pyrolysis step is carried outat a temperature of 400° C. or more.
 4. A process as claimed in claim 1,wherein the ratio of oxygen to steam in the plasma treatment step isfrom 10:1 to 2:5, by weight.
 5. A process as claimed in claim 1, furthercomprising the step of drying the waste, preferably by blowing heatedair or steam over or through it, before its treatment in the pyrolysisstep, wherein the waste is preferably dried by using the heat producedin any of the other steps of the process.
 6. A process as claimed inclaim 1, wherein the waste undergoes at least one pre-treatment selectedfrom picking, shredding, drying, screening, mixing and blending.
 7. Aprocess as claimed in claim 2, wherein the aerobic microbial digestionis carried out in an aerobic digestion unit having an oxygen content ofthe gas in the unit of not less than 5% by volume, wherein the unit ispreferably a rotary unit that rotates at a rate of from one revolutionevery minute to one revolution every ten minutes.
 8. A process asclaimed in claim 2, wherein the moisture content of the waste is atleast one of: (i) 20 to 75% by weight prior to microbial digestion; and(ii) 30% or less by weight after the microbial digestion treatment.
 9. Aprocess as claimed in claim 1, wherein the moisture content of the wasteis 20% or less by weight immediately before treatment in the pyrolysisstep.
 10. A process as claimed in claim 1, wherein the plasma treatmentof the waste is carried out at a temperature of from 1100 to 1600° C.11. A process as claimed in claim 1, wherein the plasma treatment of thewaste is carried out in the presence of a plasma stabilizing gasselected from one or more of nitrogen, argon, hydrogen, carbon monoxide,carbon dioxide and steam.
 12. A process as claimed in claim 1, furthercomprising collecting at least one of: (i) the gas produced in theplasma treatment unit; and (ii) the solid and/or molten materialproduced in the plasma treatment unit.
 13. A process as claimed in claim1, wherein the gas produced from the gas plasma treatment is used in agas engine or gas turbine to generate electricity.
 14. An apparatus forcarrying out the process as defined in claims 1, the apparatuscomprising: (i) a pyrolysis unit; (ii) a plasma treatment unit; and(iii) optionally, a unit for the microbial digestion of waste; whereinthe plasma treatment units have an inlet for oxygen and optionally aninlet for steam, and means are provided for transporting the offgas andthe non-airborne, solid char material from the pyrolysis unit to theplasma treatment unit, which is separate from the pyrolysis unit.
 15. Anapparatus as claimed in claim 14, further comprising a gas engine or gasturbine for generating electricity, the turbine being in fluidconnection with the plasma unit, so that the plasma-treated gas from theplasma unit can be fed to the turbine.
 16. A process for the treatmentof waste, the process comprising: (i) a gasification step comprisingtreating the waste in a gasification unit in the presence of oxygen andsteam to produce an offgas and a non-airborne, solid char material; and(ii) a plasma treatment step comprising subjecting the offgas and thenon-airborne, solid char material to a plasma treatment in a plasmatreatment unit in the presence of oxygen and, optionally, steam, whereinthe plasma treatment unit is separate from the gasification unit.